The X-ray image that is formed from exposure of a patient or other subject to an X-ray beam has both primary and secondary components. The primary component is obtained from attenuation of the X-ray beam as it is absorbed by tissue or other materials along the beam path. The secondary component includes scattered radiation, where radiation energy is redirected in the tissue or other materials under examination rather than absorbed. Scattered energy is a type of unwanted signal or “noise” in the image and tends to blur and obscure the image, reducing image contrast.
The schematic diagram of FIG. 1 shows how scattered energy affects image content. Incident X-ray beams 10 are directed through a subject 12 and onto a detector 20. Detector 20 can be any of a number of types of X-ray image detector, such as a detector using a photosensitive film, a storage phosphor, or a digital sensor, as represented in FIG. 1. A pixel element 16 is shown for reference. The primary image content is obtained from attenuation of X-ray beams 10, traveling in a straight line, without redirection of the incident radiation energy. Information from absorption along a line through a point P1 is then obtained at pixel element 16, corresponding to point P1 of subject 12 as desired.
In the radiation scheme of FIG. 1, secondary image information is scattered energy that is not directed from a point within the subject to the nearest pixel element, as shown at a redirected beam 18. As the path of beam 18 shows, this type of scattered energy contributes to unwanted signal; in the case of FIG. 1, this redirected energy contributes to the signal at pixel element 16. The net effect of this redirected energy on the image is thus to add unwanted signal to the image, reducing image contrast and therefore adversely impacting image quality.
Scatter is typically quantified in terms of a Scatter-to-Primary ratio (S/P) wherein S indicates the amount of X-ray signal reaching the detector as a result of scatter of the primary beam and P represents the attenuated primary beam that reaches the detector in straight-line fashion, as shown in FIG. 1.
In some types of X-ray imaging, the percentage of scattered radiation expressed by S/P can be quite high, depending on various factors such as the radiation energy level applied, the subject's width, and content characteristics. For thicker body parts, such as the sub-diaphragm area in a chest exam, for example, the S/P ratio can be as high as 20:1 without a grid. Even within the relatively radio-lucent lung region, the S/P ratio may be on the order of 2:1 without a grid.
In the typical clinical imaging situation, one common method of reducing scatter is to use a radiographic grid. The grid uses a pattern of alternating radio-opaque (lead foil) and radiolucent (for example, aluminum) strips, arranged on edge to admit radiation between the strips. The edge of these strips is turned towards the x-ray source. The spacing of the strips determines the grid frequency, and the height-to-distance between lead strips determines the grid ratio. Grids can be oriented horizontally or vertically relative to the imaging medium. In a focused grid, the strips are angled to match the divergence of the x-ray beam. This arrangement helps to optimize the radiation path for the primary image content and to reduce scatter. Both stationary and moving grids can be used.
While the use of a grid can improve image contrast, there can be drawbacks to grid use. One drawback is the need to increase the dose when the grid is deployed. Other difficulties relate to practical concerns, including the need to position the grid, which may be awkward for the patient or technician, time-critical imaging requirements that may preclude taking the extra steps for grid positioning, system setup and use, and lack of standards for grid use. Different grid frequencies and grid ratios are optimized for different types of imaging conditions, complicating the job of specifying and locating the grid that might work best for a given examination. Another drawback is the potential for grid artifacts, such as shadows and aliasing, and the need for additional processing to suppress these artifacts in some cases.
In some hospital environments, the use of a grid for chest X-rays can be a standard or recommended practice. However, it can be difficult to enforce compliance and there can be urgent situations in which standard practices can be suspended. Moreover, even when a grid is used, some amount of scattering occurs. Thus, there is still a need for approaches that address the problem of scattering and compensate for scatter without compromising the image content.
There have been a number of approaches proposed for compensating for scatter in radiographic images using computational tools.
U.S. Pat. No. 6,104,777 entitled “Process for the Correction of Scattering in Digital X-Ray Images” to Darboux et al. describes an analytical approach that employs a 3-D model of the subject to estimate primary and secondary (scattered) image content. The method computes the scattered component using an integral transformation, then subtracts the secondary image content from the image.
U.S. Pat. No. 7,551,716 entitled “Apparatus and Method for Scatter Correction in Projection Radiography” to Ruhmschopf, primarily directed to mammography, employs a set of predefined tables, generated using Monte-Carlo simulation, that characterize the spread function of the scatter component in a scatter correction scheme.
U.S. Pat. No. 4,918,713 entitled “System and Method for Correcting for Scattered X-Rays” to Honda employs a characterization of the point spread function caused by scatter, then uses frequency transformation and convolution to identity and reduce the scatter component.
U.S. Pat. No. 6,633,626 entitled “Methods and Apparatus for Correcting Scatter” to Trotter et al. uses an iterative processing method to provide a variable scatter compensation based, in part, on the thickness of subject tissue.
U.S. Pat. No. 5,440,647 entitled “X-ray Procedure for Removing Scattered Radiation and Enhancing Signal-to-Noise Ratio (SNR)” to Floyd, Jr. et al. describes the use of a statistical estimation technique and an iterative algorithm that progressively removes image content that is due to scatter effects.
U.S. Pat. No. 5,615,279 entitled “Method of and Apparatus for Correcting Scattered X-rays for X-ray Computerized Tomograph” to Yoshioka et al. describes scatter correction using modeled data.
Approaches described in these disclosures include computational complexity and the need for substantial computer resources. This is a particular problem in urgent cases, where there may not be sufficient time for execution of full-blown image processing for scatter correction as taught in a number of the approaches listed previously. A further shortcoming of these solutions relates to the need for contrast uniformity, including providing some measure of consistent rendering for similar images, whether or not a radiographic grid is employed.
Thus, it can be seen that there is a need for scatter compensation that does not require extensive computer time and resources and that provides an improved measure of consistent rendering for presenting radiographic images.